1. Field of the Invention
This invention relates to an electron source and an image-forming apparatus such as a display apparatus incorporating an electron source and, more particularly, it relates to a novel surface conduction electron-emitting device as well as a novel electron source and an image-forming apparatus such as a display apparatus incorporating such an electron source.
2. Related Background Art
There have been known two types of electron-emitting devices; the thermoelectron type and the cold cathode type. Of these, the cold cathode type includes the field emission type (hereinafter referred to as the FE-type), the metal/insulation layer/metal type (hereinafter referred to as the MIM-type) and the surface conduction type.
Examples of the FE electron-emitting device are described in W. P. Dyke & W. W. Dolan, “Field Emission”, Advances in Electron Physics, 8, 89 (1956) and C. A. Spindt, “PHYSICAL Properties of thin-film field emission cathodes with molybdenum cones”, J. Appl. Phys., 47, 5284 (1976).
MIM devices are disclosed in papers including C. A. Mead, “The tunnel-emission amplifier”, J. Appl. Phys., 32, 646 (1961). Surface-conduction electron-emitting devices are proposed in papers including M. I. Elinson, Radio Eng. Electron Phys., 10 (1965).
An SCE device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson proposes the use of SnO2 thin film for a device of this type, the use of Au thin film is proposed in G. Dittmer: “Thin Solid Films”, 9, 317 (1972) whereas the use of In 2O3/SnO2 and that of carbon thin film are discussed respectively in M. Hartwell and C. G. Fonstad: “IEEE Trans. ED Conf.”, 519 (1975) and H. Araki et al.: “Vacuum”, Vol. 26, No. 1, p. 22 (1983).
FIG. 27 of the accompanying drawings schematically illustrates a typical surface-conduction electron-emitting device proposed by M. Hartwell. In FIG. 27, reference numeral 1 denotes a substrate. Reference numeral 2 denotes an electrically conductive thin film normally prepared by producing an H-shaped thin metal oxide film by means of sputtering, part of which eventually makes an electron-emitting region 3 when it is subjected to an electrically energizing process referred to as “electric forming” as described hereinafter. In FIG. 27, the thin horizontal area of the metal oxide film separating a pair of device electrodes has a length L of 0.5 to 1 mm and a width W of 0.1 mm. Note that the electron-emitting region 3 is only schematically shown because there is no way to accurately know its location and contour.
As described above, the conductive film 2 of such a surface conduction electron-emitting device is normally subjected to an electrically energizing preliminary process, which is referred to as “electric forming”, to produce an electron emitting region 3. In the electric forming process, a DC voltage or a slowly rising voltage that rises typically at a rate of 1 V/min. is applied to given opposite ends of the conductive film 2 to partly destroy, deform or transform the conductive film and produce an electron-emitting region 3 which is electrically highly resistive. Thus, the electron-emitting region 3 is part of the conductive film 2 that typically contains fissures therein so that electrons may be emitted from those fissures. The thin film 2 containing an electron-emitting region that has been prepared by electric forming is hereinafter referred to as a thin film 4 inclusive of an electron-emitting region. Note that, once subjected to an electric forming process, a surface conduction electron-emitting device comes to emit electrons from its electron-emitting region 3 whenever an appropriate voltage is applied to the thin film 4 inclusive of the electron-emitting region to make an electric current run through the device.
Known surface conduction electron-emitting devices having a configuration as described above are accompanied by various problems, which will be described hereinafter.
Since a surface conduction electron-emitting device as described above is structurally simple and can be manufactured in a simple manner, a large number of such devices can advantageously be arranged on a large area without difficulty. As a matter of fact, a number of studies have been made to fully exploit this advantage of surface conduction electron-emitting devices. Applications of devices of the type under consideration include charged electron beam sources and electronic displays. In typical examples of applications involving a large number of surface conduction electron-emitting devices, the devices are arranged in parallel rows to show a ladder-like shape and each of the devices are respectively connected at given opposite ends with wirings (common wirings) that are arranged in columns to form an electron source (as disclosed in Japanese Patent Application Laid-open Nos. 64-31332, 1-283749 and 1-257552). As for display apparatuses and other image-forming apparatuses comprising surface conduction electron-emitting devices such as electronic displays, although flat-panel type displays comprising a liquid crystal panel in place of a CRT have gained popularity in recent years, such displays are not without problems. One of the problems is that a light source needs to be additionally incorporated into the display in order to illuminate the liquid crystal panel because the display is not of the so-called emission type and, therefore, the development of emission type display apparatuses has been eagerly expected in the industry. An emission type electronic display that is free from this problem can be realized by using a light source prepared by arranging a large number of surface conduction electron-emitting devices in combination with fluorescent bodies that are made to shed visible light by electrons emitted from the electron source (see, for example, U.S. Pat. No. 5,066,883).
In a conventional light source comprising a large number of surface conduction electron-emitting devices arranged in the form of a matrix, devices are selected for electron emission and subsequent light emission of fluorescent bodies by applying drive signals to appropriate row-directed wirings connecting respective rows of surface conduction electron-emitting devices in parallel, column-directed wirings connecting respective columns of surface conduction electron-emitting devices in parallel and control electrodes (or grids arranged within a space separating the electron source and the fluorescent bodies along the direction of the columns of surface conduction electron-emitting devices of a direction perpendicular to that of the rows of devices (see, for example, Japanese Patent Application Laid-open No. 1-283749).
However, little is known about the behavior in vacuum of a surface conduction electron-emitting device to be used for an electron source and an image-forming apparatus incorporating such an electron source and, therefore, it has been desired to provide surface conduction electron-emitting devices that have stable electron-emitting characteristics and hence can be operated efficiently in a controlled manner. The efficiency of a surface conduction electron-emitting device is defined for the purpose of the present invention as the ratio of the electric current running between the pair of device electrodes of the device (hereinafter referred to device current If) to the electric current produced by the emission of electrons into vacuum (hereinafter referred to emission current Ie). It is desired to have a large emission current with a small device current.
The inventors of the present invention who have long been engaged in the study of this technological field strongly believe that contaminants excessively deposited on and near the electron-emitting region of a surface conduction electron-emitting device can deteriorate the performance of the device, that contaminants are mainly decomposition products of oil in the evacuation system used for the device and that such deterioration can be prevented if the electron-emitting region is controlled in terms of shape, material and composition.
Thus, a low electricity consuming high quality image-forming apparatus typically comprising an image-forming member of fluorescent bodies can be realized if there provided a surface conduction electron-emitting device that has stable electron-emitting characteristics and hence can be operated efficiently in a controlled manner. Such an improved image-forming apparatus may be a very flat television set. A low energy consuming image-forming apparatus may require less costly drive circuits and other related components.